Processes for removing oxide from the surface of metals



Patented Feb. 26, 1946 PROCESSES FOR REMOVING OXIDE FROM THE SURFACE OFIVIETALS Harry R. Spence and Horace W. Hooker, Lewiston, N. Y.,assignors to Hooker Electrochemical Company, Niagara Falls, N. Y., acorporation of New York No Drawing. Application June 2, 1944, Serial No.538,538

8 Claims.

Our invention relates more particularly to processes for removing oxidefrom the surface of metals, and especially the ferrous metals, alsocopper, and their alloys, which have been or are in the process of beingrolled out into sheets or drawn into tubes, wires or other shapes, atmore or less elevated temperature. This work generally involves heatingthe metal and passing it several times through the rolls, attemperatures sufliciently elevated to soften the metal. At thesetemperatures metal picks up oxygen from the air and forms the darkoxide. Ordinarily, this oxide readily scales off; but in the course ofthe working above described, the metal becomes toughened and hardenedand the oxide film rolled into it until, in the case of the ferrousmetals and their alloys, it may form a dense coal black glaze. There mayalso be several finishing passes through dies at lower temperatures,such as result from the work of friction and deformation. For thispurpose it is necessary to remove the glaze and at intervals also toanneal the metal, in order to proceed. Removal of the glaze isdiflicult, especially in the case of the alloy steels. Annealing afterremoval of the glaze reoxidizes the metal.

Among the ferrous metals that may be treated by our process, besidesiron, are the other metals of the ferrous group, including nickel andcobalt, and their alloys with each other and with chromium, manganese,molybdenum, tungsten, vanadium and copper, including the stainlesssteels containing nickel and chromium and alloys of nickel withchromium, such as Nichrome and Tophet, which may contain little or noiron, also alloys of nickel with copper, such as Monel, which containsno iron.

Two methods in general have been used or proposed for removing oxidefrom the surface of such metals and alloys, namely:

(a) The oxide may be removed by the purely chemical action of an aqueousacid, convertin it into a soluble salt. This leaves the surface bright,but as the metal itself is also attacked by the acid, there is anappreciable loss of metal and the surface is left pitted. Moreover, thetime required for the operation is excessive, e. g., several hours.

(b) The oxide may be reduced to the metallic state. This is done bycathodic electrolysis, generally in an electrolyte of fused alkali, inwhich the metal serves as cathode. The reduced oxide may form a spongelayer on the surface of the metal, or it may fall off into theelectrolyte, where it accumulates as sludge.

- current over the surface of the metal because of electrical resistanceat the contacts, and especially through the oxide, and unequal distancesfrom the anode of the various parts of the metal pieces undergoingtreatment. This is particularly true when the metal is in the form ofshaped articles, such as stampings, coils of wire or tubes havingrecesses or interior surfaces. The difliculty of current distribution isespecially great in the case of the alloy steels, particularly thosecontaining nickel or chromium or both, owing to high electricalresistance of these alloys and variations in their oxides. Non-uniformcurrent distribution tends to localize the electrolytic action.

On this account it has sometimes been found advantageous to subject. themetal to a brief anodic treatment prior to the cathodic treatment. Theeffect of this treatment may be increased by the addition of asubstantial proportion of a chloride such as sodium chloride to thefused bath. A typical treatment of this nature would be an anodicoxidation of 30 seconds followed by a cathodic reduction of 2 minutes,in a bath of fused caustic soda containing 20 per cent sodium chloride,at 500 to 550C. and a current density of 100 to 150 amperes per squarefoot of surface of the metal under treatment. In the case of stainlesssteel, and in particular that containing 18 per cent chromium and 8 percent nickel, known as 18-8 stainless steel, this treatment leaves thesurface in a. rough condi-- tion, due to adherence of the metal left byreduction of the oxide. Such a surface is objectionable from the pointof view of appearance and also in that it renders further rolling ordrawing diflicult. In order to remove this rough coating it is sometimesnecessary to immerse 1 the metal in a strong acid, such as aqua regia,for several minutes. This drastic acid treatment gives the metal abrilliant sheen, but un-- der the microscope the surface is seen to bepitted, due to chemical attack on the metal itself. We have now foundthat after the above described anodic treatment a soaking in the fused"caustic bath, without application of electric cur- REISSUED JUN 8 I947eliminates the harmful drastic acid treatment. Even without the anodictreatment or any electrolysis at all, a soaking in the fused causticbath changes the character of the oxide so that much of it may beremoved by acid without injury to the underlying metal surface. However,the removal of oxide is promoted by the anodic treatment.

We have also found that an effect equal or superior to that of theanodic treatment may be secured by subjecting the metal to the action ofan anhydrous oxidizing agent that is effective without electrolyticdecomposition, such as an alkali metal or alkaline earth nitrate,chlorate, peroxide, dichromate, manganate, permanganate or persulphate,without any electrolysis. The oxidizing agent may be added to the fusedbath. Thus the oxidation and caustic soaking steps become simultaneousand a single operation.

In its preferred embodiment, therefore, our process consists in soakingthe article in a bath of fused caustic alkali containing 1 to 20 percent, and preferably 5 to per cent, of an active oxidizing agent, at atemperature between 300 and 600 C., and preferably at about 500 0.,depending upon the oxidizing agent, for 1 to 20 minutes, the timedepending on caustic alkali and oxidizing agent used and upon theproportion of oxidizing agent in the bath, and then dipping it briefly,e. g. for to 60 seconds, in a di- 39 lute nonoxidizing acid, such as 1to per cent hydrochloric acid, preferably at 65 to 85 C. It is desirableto wash off excess caustic alkali before the acid dip. A convenient wayof doing this is to quench the metal in water as it comes from the fusedbath. when this is done, much of the oxide is found as a sludge in thewater; also there is less consumption of acid. The electrolytictreatment, with all its difficulties of current distribution. is therebycompletely eliminated.

Our treatment changes the physical character of theoxide so that theglaze is pufled out into a coating of fine grain, which may resemblelamp black. In some cases, however, the coating is brown and if thetreatment is continued the brown oxide eventually becomes so puffed outand loosened that some of it may be shaken off. The

resulting powder is ofthe fineness of paint pigment. The extremefineness of this oxide probably accounts for the readiness: with whichit is attacked by the dilute acid. The removal of this oxide by the aciddip leaves a frosted or mat surface, of the kind that is preferred forfurther drawing, and of a color from steel grey to silvery white,depending on the nature of the steel.

When the metal is stainlesssteel we have found that after use the bathcontains sodium chromate in solution. This might be thought to indicatethat the chromium is oxidized, the oxide forming with the caustic sodathe sodium salt of chromium. which then dissolves oil in the bath.

Our fused alkali bath may consist of caustic soda, caustic potash or amixture of these alkalies with each other or with their carbonates orcalls-- tic lime. The presence of the oxidizing agent lowers the meltingpoint of the bath. Thus, 10 per cent of sodium nitrate lowers themelting point of caustic potash from 360 to 315 C. Also, by properproportioning of a mixture of alkalies, it is possible to produce a bathof substantially lowor melting point than that of any one of itsindividual components. Thus a mixture of commercial caustic soda andcaustic potash in equal proportions by weight melts at 205 0., whereasthese alkalies, in commercial grades, melt at 325 and 360 C.respectively. Such a low melting bath will have in higher degree theproperty of wetting or soaking into the oxide and by using one of themore active oxidizing agents, such as the alkali metal and alkalineearth chlorates. may be operated at lower temperature than a bath of asingle caustic alkali. Naturally the favorable proportions for suchmixture include their eutectics.

Our process therefore effects not only a great simplification andsubstantial cheapening of the operation, as compared with earlierprocesses involving one or more electrolytic treatments including afinal cathodic reduction followed by a drastic treatment with strongacid, but also a substantial improvement in the result.

We have also found that when further drawing through dies at lowertemperatures is required, the coating produced by our process need notbe removed until after the final drawing. In that case, it serves as avery effective base for the lubricant for the dies during the next fouror five successive operations, perhaps because the particles of oxideare so fine that they roll. This makes it possible to dispense with thelead coating generally applied for lubrication purposes. After the finalworking the metal is given the quick acid dip, which leaves it with thefrosted surface above described.

Although we prefer to add the oxidizing agent to the fused causticalkali bath, it may, if preferred, be used by itself in a separate steppreliminary to the treatment in the fused caustic alkali bath. In thiscase the choice of oxidizing agents is increased.

Example I A drawn rodof "18-8 stainless steel having a dark glazedsurface was immersed for 4 min- 40 utes in a bath of fused caustic sodacontaining 10 per cent sodium nitrate by weight at 575 C. When removedthe surface coating had been converted to a condition resembling lampblack. A water wash and dip of 30 seconds in dilute hydrochloric acidremoved the oxide completely, leaving a clean frosted silvery greysurface.

Example 11 A piece of the same rod as that of Example I was treated inthe same way except that the temperature was 500 C. and the time 5minutes. The result was substantially the same. This at presentrepresents preferred practice.

Example III A piece of the same rod as that of Example I was treated inthe same way except that the temperature was 420 C. and the time 15minutes. The result was substantially the same. A comparison of thisexample with the two preceding shows that the time of treatment in ourprocess is a function of the temperature of the bath.

Example IV 1 aromas- Examplc V Another piece of the same rod wasimmersed verted to a coating resembling reddish brown paint pigment. Awater 'wash and acid dip of one minute removed the brown coating.leaving a clean frosted surface.

Example VI Another piece of the same rod as that treated in Example Iwas immersed in a bath of fused caustic potash containing per centsodium nitrate by weight for 4 minutes at 420 C. It

came out with the oxide converted to a brown coating resembling that ofExample V. The water wash and acid dip removed this, leaving a cleanfrosted surface, though not quite so bright as that of Example I;probably due to the time not having been quite long enough.

Example VII Another piece of the same rod was immersed for 6 minutes ina bath of fused caustic potash containing 10 per cent of potassiumchlorate by weight at 420 C. It came out with the oxide converted to acondition resembling that of Example V. The water wash and acid dip leftthe surface frosted and silvery white, brighter than in any of thepreceding examples, showing that the chlorates are very effectiveoxidizing agents for the purpose of our process.

Example VIII Another piece of the same rod was immersed in a fused bathof caustic soda and caustic potash in equal proportions by weight,without any oxidizing agent, at 400 C., for 10 minutes. It came out avery dark brown in color. The wash and acid dip removed a part but notall of the oxide, showing that in the absence of the oxidizing agent theeffect is less positive.

Example IX Another piece of the same rod was immersed in a bath of fusedsodium nitrate alone for 6 minutes and given a water wash and acid dip.It was little aifected. This is believed to show that the caustic alkaliis essential.

Example X Another piece of the same rod was immersed for 5 minutes. infused sodium nitrate, then for 5 minutes in a bath of fused caustic sodaat 415 0., followed by a water wash and dip in dilute acid. The resultwas similar to that of Example 111, showing that the treatments withoxidizing agent and caustic soda may be simultaneous or successive.

Example XI tion of these salts and any preliminary reduction,-

Example XII A badly rusted cast-iron pipe elbow was treated as inExample XI. It came out with the lamp black like coating. After the washand acid dip it was the grey color of clean cast iron.

Example XIII A piece of old badly rusted steel pipe was treated as inExample XI with the same result.

Example XIV A strip of Nichrome containing per cent nickel and 20 percent chromium and no iron, heavily coated with dark oxide, was immersedin the same bath as in Example III for the same time and at the sametemperature. It came out with a reddish brown irridescent coating whichwas completely removed by the water wash and acid dip, leaving abeautiful shiny surface.

Example XV A sheet of "MoneP coated with dark oxide was immersed in afused bath of caustic soda containing 6 y per cent sodium nitrate at 500C. for 5 minutes. It came out with the oxide converted to a coatingresembling lamp black. It was quenched in water and immersed in warm 10per cent hydrochloric acid for one minute. The surface was left abeautiful silvery white.

Example XVI Pieces of cast and stamped copper heavily coated with amottled reddish brown oxide were treated as in Example III. They cameout quite black. After the wash and acid dip their surfaces wereimmaculately clean, that of the casting appearing frosted.

Although we have emphasized treatment of alloy steels, that is onlybecause they present the greatest difliculty. A large field ofapplication if of course that of ordinary low or high carbon steels. Theforegoing examples show that Our process is also applicable to copperand, in general, to any metal or alloy that is resistant to causticalkalies under the conditions of treatment, and preferably up to atleast 600 C. Our process is applicable to castings as well as toarticles formed from the malleable metal by stamping, rolling, drawing.etc. It is also applicable to articles that have become oxidized at lowtemperature, as by atmospheric oxidation.

Although in the foregoing specification and examples we have indicatedthat the acid should be non-oxidizing and have used only dilutehydrochloric acid as an illustration of such an acid, we do not-wish tobe limited thereto. Other acids, including nitric and sulphuric acid,which are ordinarily considered as oxidizing acids, may be used if insuch dilution and at such a temperature that they do not have anoxidizing effect or otherwise attack the metal.

While we do not wish to be held to any particular theory as to thereactions that take place in our process, it would seem that the fusedcaustic alkalies form the corresponding salts of the metal oxides, whichare generally black but sometimes a very distinctive reddish brown, andthat these salts being basic as well as quasi-molecular, are veryreactive with acids. It appears that the presence of the oxidizing agentpromotes formaas by cathodic electrolysis, hinders their formation. Thepresence of any considerable quantity of water would prevent thenecessary temperature from being reached and perhaps operate adverselyin other ways.

We claim as our invention:

1. The method of removing from the surface of metals resistant to fusedcaustic alkalies dense firmly adherent oxide that has formed thereonthrough exposure to air under oxidizing conditions which comprisessubjecting the oxide to the action ofa fused substantially anhydrousbath comprising caustic alkali containing a substantial but minorproportion of an oxidizing agent stable therewith, said bath beingmaintained at an oxidizing potential with respect to the metal,

at a temperature of 300 to 600 C., for not less than 1 minute, and thensubjecting the oxide to the action of a weak inorganic acid.

action of a fused substantially anhydrous bath,

comprising caustic alkali containing a substantial but minor proportionof an oxidizing agent stable therewith, said bath being at substantiallythe same electrical potential as the metal, at a temperature of 300 to600 C., for not less than 1 minute, and then subjecting the modifiedoxide to the action of a weak aqueous inorganic acid.

3. The method of removing from the surface of metals that have beentreated by the process or claim 2 oxide that has formed thereon throughsubsequent exposure to air at annealing temperture which comprisessubjecting the oxide to the action of a fused substantially anhydrousbath comprising caustic alkali containing a substantial but minorproportion of an oxidizing agent stable therewith, at a temperature of300 to 600 C'., and then subjecting the a weak inorganic acid.

4. The method of removing from the surface of metals resistant to fusedcaustic alkalies dense firmly adherent oxide that has formed thereonthrough exposure to air under oxidizing conditions which comprisessubjecting the oxide to the action or a fused substantially anhydrousbath, comprising at least one caustic alkali of the group consisting ofsodium, potassium and calcium hydroxides and a substantial but minorproportion of at least one oxidizing agent of the group consisting ofthe alkali metal and alkaline earth chlorates, peroxides, nitrates,dichromates,

oxide to the action of manganates, permanganates and persulphates,

said bath being at substantially the same electrical potential as themetal, at a temperature of 300 to 600 C fo r not less than 1 minute, and

then subjecting the modified oxide to the action of a weak aqueousinorganic acid.

5. The method of removing from the surface of metals of the groupconsisting of iron, nickel and cobalt and their alloys with each otherand with chromium, manganese, molybdenum, tungsten, vanadium and copper,dense firmly adher ent oxide that has formed thereon through exposure toair under oxidizing conditions which comprises subjecting the oxide tothe action of a fused substantially anhydrous bath, comprising causticalkali containing a, substantial but minor proportion of an oxidizingagent stable therewith, said bath being at substantially the sameelectrical potential as the metal at a temperature of 300 to 600 6., fornot less than 1 minute, and then subjecting the modified oxide to theaction of a weak aqueous inorganic acid.

6. The method or removing from the surface of chrome-nickel "stainlesssteel dense firmly adherent oxide that has formed thereon throughexposure to air under oxidizing conditions which comprises subjectingthe oxide to the action of a fused substantially anhydrous bath,comprising caustic soda containing 5 to 15 per cent 01 sodium nitrate,said bath being at substantially the same electrical potential as themetal, at 400 to 600 0., for not less than 1 minute, and subjecting themodified oxide to the action 01' 5 to 15 per cent aqueous hydrochloricacid.

I. The method of removing from the surface of chrome-nickel stainlesssteel dense firmly adherent oxide that has formed thereon throughexposure to air under oxidizing conditions which comprises subjectingthe oxide to the action of a fused substantially anhydrous bath,comprising caustic soda containing 5 to 15 per cent of potassiumchlorate, said bath being at substantially-the same electrical potentialas the metal, at 400 to 600 C., for not less than lminute, andsubjecting the modified oxide to the action of 5 to 15 per cent aqueoushydrochloric acid.

8. The method of removing from the surface of chrome-nickel "stainlesssteel" dense firmly adherent oxide thathas formed thereon throughexposure to air under oxidizing conditions which comprises subjectingthe oxide to the action of a fused substantially anhydrous bath,comprising caustic soda containing 5 to 15 per cent of sodium chlorate,said bath being at substantially the same electrical potential as themetal, at 400 to 600 C., for not less than 1 minute, and subjecting themodified oxide to the action of 5 to 15 per cent aqueous hydrochloricacid.

HARRY R. SPENCE. HORACE W. HOOKER.

